Re: [PATCH v2] riscv: entry: always initialize regs->a0 to -ENOSYS

On Thu, Jun 27, 2024 at 10:23:39PM GMT, Celeste Liu wrote:

Otherwise when the tracer changes syscall number to -1, the kernel fails to initialize a0 with -ENOSYS and subsequently fails to return the error code of the failed syscall to userspace. For example, it will break strace syscall tampering.

Fixes: 52449c17bdd1 ("riscv: entry: set a0 = -ENOSYS only when syscall != -1") Reported-by: "Dmitry V. Levin" ldv@strace.io Reviewed-by: Björn Töpel bjorn@rivosinc.com Cc: stable@vger.kernel.org Signed-off-by: Celeste Liu CoelacanthusHex@gmail.com

---

arch/riscv/kernel/traps.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c index 05a16b1f0aee..51ebfd23e007 100644 --- a/arch/riscv/kernel/traps.c +++ b/arch/riscv/kernel/traps.c @@ -319,6 +319,7 @@ void do_trap_ecall_u(struct pt_regs *regs)

regs->epc += 4;
regs->orig_a0 = regs->a0;

• regs->a0 = -ENOSYS;
  
  

  riscv_v_vstate_discard(regs);

@@ -328,8 +329,7 @@ void do_trap_ecall_u(struct pt_regs *regs)

if (syscall >= 0 && syscall < NR_syscalls)
	syscall_handler(regs, syscall);

• else if (syscall != -1)
  

• 	regs->a0 = -ENOSYS;
  

• /*
  • Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
  • so the maximum stack offset is 1k bytes (10 bits).

Hi,

this change seems to have broken strace's test suite.

In particular, the "legacy_syscall_info" test, which is meant to verify that strace behaves correctly when PTRACE_GET_SYSCALL_INFO is not available, reports a bogus value for the first argument of the syscall (the one passed via a0).

The bogus value comes directly from the ptrace() call, before strace has a chance to meddle with it, hence why the maintainer suggested that the issue would likely be traced back to the kernel.

I have built a kernel with this change reverted and, as expected, the strace test suite passes. Admittedly I've used the 6.11-rc7 Fedora kernel as the baseline for this test, but none of the Fedora patches touch the RISC-V code at all and the file itself hasn't been touched since rc7, so I'm fairly confident the same behavior is present in vanilla 6.11 too.

See

https://github.com/strace/strace/issues/315

for the original report. Please let me know if I need to provide additional information, report this anywhere else (bugzilla?), and so on...

Thanks!